Disc brake pads are to brakes what a camshaft is to an engine. Where a cam is critical for defining an engine's power potential, brake pads are vital for successfully and repeatedly decelerating a vehicle. Experienced hot rodders know there are a ton of variables when it comes to cam selection, with no one grind being the best for every situation. Likewise, brake engineers must juggle system wear characteristics, operating temperature range, harshness, dust, noise, environmental factors, material and manufacturing costs, and even individual driver preferences. For the serious racer, brake pad choice is a continually evolving process, changing on a nearly monthly basis.

2/14Just as an engine must be built with an optimum torque curve for the rpm range in which it will operate, a brake pad must be designed to generate maximum friction efficiency and response in its normal temperature operating range. As they do with engines, major brake companies test their designs on a dyno, as seen here at Ferodo.

A Work of Friction
The higher a brake pad's coefficient of friction (expressed by the Greek letter µ, pronounced mu), the more aggressive the pad and the greater its stopping potential (the same amount of pedal force provides more stopping power). In theory, µ ranges from 0 (full lubricity, no friction at all) to 1.0 (solid, no molecules moving). Fifteen to 20 years ago, Baer brakes says, street brake pads were lucky to see the high 0.20s. Today, even OEM pads are well into the 0.30s, top-tier performance street pads in the 0.40 to 0.45 range, and some race cars in the high 0.60s.

A pad's friction is not constant, varying due to changes in temperature, humidity, wear, age, and many other factors. Pad designers strive to develop more stable pad compounds that maintain consistency over a wide range of operating conditions. But as compounds become more exotic, they also become more expensive. And more aggressive (higher µ) compounds may increase rotor wear. There's no free lunch.

3/14Like other major companies, Raybestos thoroughly tests its brake designs in major racing venues. What's learned in major-league racing helps improve Sportsman and road-car brake technology. Here, a Raybestos integrated caliper-rotor-pad system for NASCAR cup racing is installed on a Joe Gibbs car.

The friction performance of pads intended for typical street temperatures is classified under SAE Standard J866, expressed as a two-letter code where the first letter designates the low-temperature (0 to 200 degrees F) friction performance and the second letter the high-temperature (200 to 600 degrees F) performance. The letters generally appear on the backing plate as a prefix or suffix to the part number. If the first letter is lower than the second, the pad works better at high temperatures and needs a warm-up; if the second letter is lower than the first, the pad may fade at high temperatures. The best street pads have good friction at both high and low temperatures (ideally, both letters would be the same, as in FF).

Friction Coefficient

Code

Up to 0.15 µ

C

Over 0.15 µ up to 0.25 µ

D

Over 0.25 µ up to 0.35 µ

E

Over 0.35 µ up to 0.45 µ

F

Over 0.45 µ up to 0.55 µ

G

Over 0.55 µ

H

Unclassified

Z

Not all pads are marked this way. Full-race pads may operate at high temperature ranges outside of the classification range. And some manufacturers use proprietary classifications, such as color-coding the pads or their own numbering schemes.

Friction and operating temperature differences are achieved by varying pad material. Today's commonly available pad compounds can be broadly divided into three categories: organic, sintered metal, and exotics. Manufacturers even blend materials and characteristics from different pad types to achieve specific performance goals, but be aware that some may use various proprietary trade names for marketing purposes that confuse the materials issue.

Organics
Organic compounds are lining materials that are bonded with an organic resin. They are compatible with common cast-iron discs or (as used on some motorcycles) steel discs. Organic pads can be further subdivided into semimetallic, NAO (nonasbestos organic), ceramic, or low steel compounds.

4/14One noise-reduction strategy for aggressive friction pads is attaching thin stainless steel plates (sometimes with a thin layer of rubber) to the backing plate for damping out squeal. Shims can also serve as an additional thermal barrier. Shims may be clipped, spring-locked, orbitally riveted, or (as with this Raybestos shim) thermally-bonded to the plate.

When asbestos was phased out in the '70s and '80s for health reasons, it was initially replaced by semimetallic compounds. Semimets contain a high percentage of ferrous materials like iron or steel powder that are relatively stable at elevated temperatures with good wear characteristics. Depending on the specific compound, semimets can develop significantly higher friction than old-school organic compounds. But like steel wool, semimets are abrasive, so-although the pads are long-lived-they tend to increase rotor wear on street cars, promote brake squeal, and even lead to judder and roughness issues.

More recently, NAO compounds were introduced. Instead of asbestos, their formulation is based on carbon, Kevlar, aramid or Twaron fibers, sometimes with nonferrous particulates like copper added for lubricity. They may be reinforced with fiberglass. The OEMs love NAO compounds because they're easy on brake rotors and have great resistance to noise and vibration. But the NAO pads themselves usually wear at a faster rate than semimets and aren't as stable at high temperatures. Friction characteristics vary depending on specific compounds; µ is generally (but not always) lower than the semimets. Brake dust buildup can be an issue with some (but not all) NAO formulations.

5/14Pad materials are always a compromise. This graph summarizes ECS tests on standard parts-store NAO/ceramic and semimetallic pads versus an upper-tier, high-perf, semimet. Different characteristics are ranked on a 1 to 10 scale-higher numbers mean better performance in each category (for example, greater friction or less squeal garners a higher score).

Yet another branch of the organic family tree are so-called ceramic brake pads. These are not true, exotic ceramic-composite pads (we'll get to those later); instead they are conventional pads with some amount of ceramic dust added to replace the traditional NAO compounds. Ceramic pads may or may not contain ferrous or steel elements. If they do, some manufacturers refer to this variation as a low-metallic or low-steel pad. Because of such widely varying characteristics, it's hard to make a blanket statement as to where these compounds fit in the overall scheme of things. Generally, in the absence of ferrous elements, a ceramic pad should generate higher friction values and longer wear rates with better temperature resistance than other NAO formulations. According to Wilwood, the wear dust is very clean with minimal discoloring of alloy wheels. If the ceramic pad contains ferrous elements, the result is, says Ferodo, a high-friction pad with excellent performance modulation and good life at elevated temperatures-but at the cost of increased noise.

6/14Pad materials have a graph for friction and heat that looks like an engine dyno curve. Designed to work at moderate temperatures, these Wilwood pads are for high-perf street, street/strip, and selected competition apps. BP-10 and BP-20 are Smart Pads that combine a ceramic's low noise and dust characteristics with the higher temperature ranges normally found in ferrous metallics. Compounds E and Q are PolyMatrix formulations.

Sintered Metal
Sintering is a process where metal powders are pressed together at high temperatures, rather than being bonded together by a resin like organics. Sintered materials are almost always based on copper, which during the process alloys with tin (to form bronze) or zinc (to form brass). Less commonly, iron is used as the base material. Graphite and ceramic components may also be added to moderate friction performance. Compared with organic materials, sintered pads exhibit higher durability and longer life under racing conditions. They have a very high friction coefficient, but friction modulation-a measure of consistent, linear response to increasing brake-pedal input-can be poor. Sintered metal is tough on discs, which often must be made from stainless steel to have any chance of long-term survival. Low-temp response may be poor with significant noise and dust levels. At present, this material class is generally best left to motorcycles or hard-core auto racers.

Exotics
There are also specialized exotic compounds and materials that at present remain too costly for common use. They may even be banned by some race sanctioning bodies because of their expense. Exotics include real ceramic-composite brakes. Not just an enhanced NAO or semimetallic compound, these brakes have true ceramic-infused rotors and pads. The advantage is greatly increased heat capacity, ultralight weight, and virtually no corrosion tendencies. They have exceptional fade resistance and good friction coefficients. The lighter weight offers the potential for improved handling and fuel economy benefits. Wear characteristics are exceptional as well. The downside is the manufacturing cost. So far, they've been seen mainly on high-end sports cars, including some Porsches and the '09 ZR1 Corvette.

7/14NRS is gaining traction, but on most pads the friction material is still extruded through backing plate holes and integrally molded to the backing plate with a high-temp chemical agent, as on this Wilwood intermediate-level pad. This method is vastly stronger with higher shear resistance than the relatively primitive glues used 10 or 15 years ago.

Ultimately, there are carbon/carbon-fiber brakes as used on Formula I cars, Top Fuel dragsters, and Funny CarsCarbon-fiber pads must mate to compatible carbon-fiber rotors or carbon inserts in metallic rotors.

Backing Plates
Pads are attached to backing plates. Steel remains the material of choice for most applications, except weight-critical apps such as Sprint Cars, where aluminum is preferred. Steel isn't very good at insulating the caliper pistons from heat. Some high-end race pads add a thermal barrier (usually a ceramic puck or a woven matt) sandwiched between the friction material and backing plate to resolve this problem.

8/14For hard-core rodders, Raybestos Advanced Technology pads are widely available at retail outlets. Extensively tested, they're offered in ceramic (with a P suffix at the end of the basic part number) or semimetallic (M suffix). Available for selected vehicles, the ultimate upgrade is its semimet, certified police pads (P suffix) with improved pedal feel and high-temp performance.

Pad/Rotor Interface
The pad material and rotor configuration must be compatible at operating temperatures. Rotors act like radiators when it comes to brake system heat absorption, so it is important to match rotor characteristics to friction material characteristics. For example, radically increasing rotor size might require a less aggressive, lower-temperature friction compound, because the huge rotors are so efficient in radiating heat that the original friction compound never reaches proper operating temperature.

Friction can be generated by forcing the brake pads mechanically against the spinning rotor (abrasive friction) as well as via the transferring of a thin layer of brake pad material to the rotor face as temperatures rise (adherent friction). Depending on the pad and rotor formulation, the resulting transfer layer can greatly improve overall µ. Race pads are often specifically designed to maximize the effects of this transfer film technology, generating higher friction and a lower wear rate. If your race pads are generating significant adherent friction, the rotors actually get thicker with use.

Most street pads can also establish a transfer layer, but to gain maximum benefit if changing pad compounds, first lightly burnish (or wear-mate) the rotors to wipe away the original (and likely incompatible) transfer layer. Drive your car around town at moderate speeds while braking moderately. Most manufacturers strongly caution against turning new or used rotors in a brake lathe-it does more harm than good.

9/14Used in high-end drag racing, The Brake Man's full carbon-fiber pads cost $200 to $300 per rotor set; the matching carbon rotors start at $1,100 each and go up from there. Carbon brakes have a linear and predictable friction increase over a wide temperature range. Top Fuelers now get by with just one (instead of two) calipers per wheel, yet still stop the car safely even if the parachute fails.

Bedding
Beyond burnishing, there is bedding: a systematic process of repeatedly heating the brakes to the compound's designed operating temperature range under controlled (and increasingly severe) stopping conditions to form a proper transfer layer. Most brake companies claim significantly better race brake performance after bedding-in a set of new rotors and pads or (after initial burnishing to remove the previous layer) with used rotors. The specific bedding process differs for street and track use as well as with different materials and operating temperatures. Consult your brake friction compound manufacturer for the instructions that apply to a specific product. Some high-end street performance pads and race pads are said to be preburnished and/or prebedded at the factory. Prebedded pads are usually intended for use in combination with a specific rotor package; they could prove advantageous for long-distance racers who need to change pads during an endurance race.

Selection
Realistically, no one brake pad can do everything for everybody, but broadly speaking, different brake usage scenarios require a different selection of pad materials. For high-performance street use, a compound that works at low to moderate temperatures with good wear characteristics, low noise, and minimal dust is recommended. Depending on the car and pad material, this generally points toward a quality semimetallic, a semimetallic with carbon, or an improved NAO pad with ceramic content. There are some racing compounds that still perform well at low temperatures that in theory could be used on a street car, but be prepared for significantly increased rotor and pad wear. A relatively cost-effective solution that's widely available at retail outlets is Raybestos' Advanced Technology line.

10/14Baer DecelaPads are an example of today's cutting-edge street-perf brake pads. Baer terms them a "ceramic matrix pad that delivers smooth, progressive torque." Note the FF suffix at the end of the pad application number (close-up): This means the pad should generate high friction (between 0.35 and 0.45 µ) for a street pad over a wide temperature range.

According to recognized brake expert Fred Callahan, road cars with ABS (antiskid brake systems) sometimes had problems if a pad with significantly different friction characteristics from the original OEM compound was used at the front or rear of the vehicle. Older systems could sometimes activate under normal stopping conditions or become overpowered and cause wheel lockup. "But now, as the ABS systems become more sophisticated and onboard computers play a larger role in braking performance and bias, these factors have been practically mitigated," Callahan says. If you're concerned, Hawk Performance ceramic pads feature a linear friction profile that's claimed to allow ABS systems to work more effectively.

When it comes to racing, it's best to consult the manufacturer for specific advice for your car, driving style, and type of racing. However, broadly speaking, for drags, obviously there is a huge difference between a heavy door-slammer and a Top Fuel car. But one common factor is the friction material must work cold with no warm-up time. Drag racers will trade off high-temp friction and temperature performance for instant bite and good µ-all without wheel lockup. For bracket and many Sportsman cars, Stainless Steel Brakes points out, "Generally a good performance street pad . . . will provide the low-temperature performance needed to stage the car while still providing the fade-free high-temperature performance needed to stop the car at the end of the run."

11/14Cobalt Friction XR-series carbon-ceramic/sintered-metal hybrid race pads have an exceptionally wide temperature range. Thermally stable under extreme temperatures, they work without bedding in. Claimed rotor wear is 50 percent better than competitors at a given friction level and temperature range. Street versions could be available by mid-year.

For oval tracks, friction characteristics and high-temperature performance are important selection factors, but so is vehicle weight, brake balance to set the car going into a corner, the overall track length, and even driver preference. Race semimetallic or low-metal compounds, often with added ceramic and carbon content, hold sway here. Long-distance road racing must balance achieving high friction with good wear rates. Generally, endurance cars will trade off ultimate friction for longevity.

Brake pedal feel or bite is also important. Most street drivers and some racers prefer constant friction: At operating temperature, as more pedal pressure is applied, µ remains constant or slightly tapers off. You must press the pedal harder to bring the car to a full stop. On the other hand, many road racers like rising friction: At operating temperature, as the brakes are applied, friction gradually increases, so the driver doesn't have to press down on the pedal harder to generate more braking as the car goes through a corner.

12/14Ferodo's DSPF pad series is marketed for street use. The low-steel formulation is a close cousin of Ferodo's DS2500 compounds designed for track day usage. The DSPF series is quieter but at the expense of a slightly longer pedal when hot. These compounds (under a different name) are OE on many late-model Vettes.

Inspection
You can see if your pads are getting the job done by periodically inspecting them. The Brake Man's William Gilliland says if the pad shows normal wear with no discoloration, it's operating properly. Taper or uneven wear can indicate a pad alignment problem and/or the caliper is overheating (consider upgrading the calipers). A "white-lip mustache near the rotor or cracking" indicates you're pushing the pad's limits. If the pad is entirely discolored, completely white, or crumbling, "you've pushed the pad beyond its thermal limit." "For all forms of racing, you should use temperature paint on the rotors and temperature decals on the calipers," Callahan adds. Also use an inexpensive infrared heat gun to record temperature values as you come off the track. That would be very useful when discussing pad choices with a brake rep."

The Future
Expect existing compounds to continue to evolve with ever-wider operating temperature ranges and longer lives. You will see increased use of improved NAO ceramic pads, true ceramic pads, and other exotics as costs come down. On the street, increasingly sophisticated ABS computers will permit more aggressive pad materials without the traditional dust and wear penalties previously associated with them. Phenol-formaldehyde, the current bonding resin for pad materials, may be superseded by silicon-based resins with increased high-temperature stability. Regenerative braking-wherein brake energy is used to help recharge the electrical system on electric cars or hybrids-may cause radical changes in future brake systems as manufacturers pull out all the stops in a quest to perfect the longest-lasting, best-stopping, widest-temperature compounds.

13/14Wilwood's PolyMatrix racing pad series combines attributes from a variety of friction elements, allowing incremental changes in friction and temperature to suit driver preference and track condition. They're said to yield a full metallic compound's extreme high friction and fade resistance while still providing excellent cool temperature response.

Anatomy of a Pad1. Friction Material - Bonded together by various resins, the friction compound on this Raybestos pad is most commonly a semimetallic or NAO ceramic formula tailored for the specific application and temperature range. The best high-perf street friction materials generate about 0.040 to 0.045 µ at operating temperature.

14/14

2. Slots - Slots offer an escape route to prevent dust from accumulating between the pad and rotor face, reduce stress crack formation by allowing thermal expansion of the friction pack at elevated temperatures, and increase µ by providing redundant leading edges across the pad face.

3. Chamfers - Standard and compound chamfer angles help the pad tune out certain noise frequencies. You may also see chamfers in some high-perf pads: By reducing the initial surface area, they increase the temp rise during initial brake application, providing a more stable µ. As the pad wears, the chamfer will, too. That's OK, because a thinner pad is inherently more stable, as its now-reduced thermal mass gets up to temperature quicker.

4. Backing Plate With NRS Hooks - Pad backing plates are usually low-carbon steel or high-tensile aluminum. No one rivets or glues pads to the backing plates any longer. Instead, they're bonded and secured using a high-temp molding process. The latest evolution is NRS (Nucap retention system), hundreds of J-shaped steel hooks that lock the extruded pad to the plate, providing even wear characteristics across the entire pad surface and preventing any delamination or edge lift. NRS is very effective at high race-pad temps.